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7 inhibition & facilitation in synapses (inhibition (presynaptic…
7
inhibition & facilitation
in synapses
inhibition
is of 2 types
presynaptic inhibition
this type of inhibition often
occurs at the presynaptic terminals
b4 the signal ever reaches the synapse
-is usually caused by dischage of
inhibitory interneurones
1.it is induced by axo-axonal synapses
here afferent fibers can influence
ech other
-by means of collaterals -which innervate
interneurons
-these then form synaptic contacts with
the presynaptic terminals
of the other afferent fibers.
the interneuron which cause presynaptic
inhibition lie at the base of the dorsal horn
2.mechanism,:
activation of presynaptic receptors
reduces the opening of Ca2+ channels directly or
incr Cl- & or K+ permeability
-so thet the size of the AP reaching the
excitatory ending is reduced
direct inhibition of neurotransmitter release
(independent of Ca2+ influx into the excitatory ending)
3.inhibitory transmitter at the axo-axonal synapse
is gamma-aminobutyric acid (GABA)
acting via GABA -A receptors
-it incr Cl- permeability
whereas GABA-B receptors via G-protein
produces an incr in K+ permeability
physiological significance
in general all types of myelinated somaesthetic
primary efferent fibers are subjected to
presynaptic inhibition,therefore:
activity in myelinated cutaneous afferents are
paticularly powerful in inducing presynaptic inhibition of the cutaneous afferents themselves
muscle afferents recieve predominantly presynaptic inhibitory influences frm other muscle afferents
feed-forward inhibition seen in the cerebellum
postsynaptic inhibition
it depends on the delivery of an
inhibitory chemical
-by the incoming axons
-which hyperpolarizes the
subsynaptic membrane
direct inhibition
(afferent inhibition)
inhibition which results due to
-stimulation of afferent nerve
which passes directly to the
motor neurons
in the spinal cord
thus,postsynaptic inhibition
during IPSP is called direct inhibition
-because its not due to the effects of
previous postsynaptic neuron discharge
eg:
Psynaptic inhibition in the spinal cord
golgi tendon organ inhibition
indirect inhibition
inhibition which occurs
due to the effects of previous
Psynaptic neuron discharge
eg:
1.the Psynaptic cell can be refractory to excitation cos it has just fired & is in its refractory period
2.during after-hyperpolarization it is less excitable
3.Renshaw cell inhibition
in this type of inhibition,neurons may inhibit themselves in a negetive feedback fashion
facilitation
leads to a longer & stronger response
in the postsynaptic cell
presynaptic facilitation
is produced when
the AP is prolonged & the
Ca2+ channels are open
for a longer period
& depolarization gets delayed
-allowing depolarization to continue
this is mediated by serotinin